The present invention relates to a search operation control apparatus in an optical disc reader. More specifically, it relates to an apparatus for search operation control in an optical disc reader which reads an optical disc, such as a compact disc or a laser disc, under constant linear velocity (CLV) control, for example, modified constant linear velocity (MCLV) control.
The compact disc (CD), a type of optical disc, has become very popular and has superseded the LP record in the field of audio reproduction systems.
An optical disc is usually driven at either a constant angular velocity (CAV) or at a constant linear velocity (CLV). In a CD system using CLV control, the speed of the disc is changed in correspondence with the position of an optical pickup along the disc, or with the radius of a track read by the pickup.
In conjunction with the more widespread used CDs, the CD as a read-only memory (CD-ROM) has come into standardization. The CD-ROM is a 12 cm-diameter CD for storing data and programs, from which a computer can read the information stored.
The CD-ROM has a large memory capacity and provides high-speed access and search capabilities. For example, a CD-ROM can store data of 7.83.times.10.sup.8 bytes, or as much as the total memory capacity of about one hundred to three hundred floppy discs of 3.5 inches. The access time for searching and reading desired data is about 0.7 seconds on average, which is about one hundred times as fast as the access time of a 10-inch magnetic tape having almost the same data storage capacity. The access time is defined therein as the sum of the seek time, in which the optical pickup is seeking a desired track, and the wait time, in which the optical pickup is awaiting the desired data on the track.
As mentioned in the foregoing, CD-ROM access time is less than that for a magnetic tape, yet it is not sufficiently less by comparison with that of other magnetic storage mediums, such as hard discs. The hard disc, for example, has an average access time of slightly more than 10 msec, which is roughly one-tenth that of a CD-ROM. Accordingly, it is desirable to shorten the access time for the CD-ROM. Shortening the access time is also desirable for CD-Interactive devices, and CD devices (for example, 6.5-cm CD device) which can be rewritten for one or more times. Furthermore, it is also desirable for optical (or optical-magnetic) disc readers or players which are loaded with a 30-cm or 20-cm CD which can be rewritten for one or more times.
The information stored in an optical disc such as a CD is read by the optical pickup. The CD has tracks comprising pits arranged in a segmental spiral. The optical pickup generates a spotting beam radiating therefrom onto the tracks of the CD and detects its reflection to thus apply output signals to a signal processor which follows.
In ordinary reading operation, an optical disc reader drives the optical pickup in the radial direction along a disc at low speed in order to permit the spotting beam to scan the spiral tracks on the CD. At the same time, a spindle motor drives the disc and is servocontrolled by synchronizing signals derived from the disc in order to maintain the linear velocity of the disc at a constant value. That is, the rotation speed of the disc is varied in proportion to the position of the optical pickup in the radial direction, or to the position of the track to be read.
When the CD reader performs a searching operation, the optical pickup is first moved at high speed in the radial direction. During the movement, the reader counts the number of tracks which the optical pickup crosses, and then it controllably moves the optical pickup in accordance with the counted number of and the information obtained from the tracks. In order to achieve the rotation speed which corresponds to a desired track position, the disc is either accelerated or decelerated, depending upon the synchronizing signals detected by the optical pickup. The disc thus can approximately attains the rotation speed which corresponds to the desired track position. Following the controllable movement of the optical pickup, the spindle to the disc is servocontrolled such that the disc achieves the rotational speed accurate for the desired track position.
To obtain constantly correct synchronizing signals from the tracks during a search operation is, however, difficult, due to the high speed at which the optical pickup crosses the tracks. This makes servocontrol of the spindle motor inexact and protracts the time for access via the optical pickup. While it may be possible to eliminate irregular portions of the synchronizing signals electronically in order to solve the problem, this method cannot eliminate all such irregularities.
Furthermore, accurately counting the number of tracks scanned during a search in order to reduce the access time may prove impossible due to count failure. Because CD tracks consist of pits arranged in a segmental spiral, as described in the above, count failure arises when the optical pickup crosses a track at a place where no pits exist.
Tracks fail to be counted particularly during a search from the inner portion towards the outer portion of a disc. That is, when the search is performed from the inner portion toward the outer portion, the disc rotates at low linear velocity as it is decelerated. Under these circumstances, a large transverse angle is formed against a crossed track by the spotting beam locus, thus the spotting beam of the pickup crosses a lesser number of pits, such that the reader may fail to count tracks. Failure to count tracks results in an increased seek time for arrival at the desired track, thereby protracting the access time.